The present invention concerns a thermoelectric element comprising at least one thermopair and a pn-junction, wherein the thermopair has a first material with a positive Seebeck coefficient and a second material with a negative Seebeck coefficient and a thermoelectric generator and a thermoelectric cooler with a thermoelectric element of that kind.
The state of the art is divided into various areas which involve different development stages.
The thermoelectric effect has already been known for more than 100 years, and there is a wide range of materials which can be used for the direct conversion of a temperature difference into electric current (thermoelectric generator) or for cooling upon the application of an external voltage source (thermoelectric cooler). Technical implementation of the generator effect has hitherto always been based on a common fundamental structure (FIG. 6). Two different metals or two differently doped (n- and p-doped) semiconductors are connected to an end, in the normal case the hot end, by way of a metallic conductor, and then the current can be taken off at the other end, which is normally the cold end. The energy for overcoming the differences in the electrical potential between the materials at the contact locations is taken from the ambient heat (Peltier effect).
In order to achieve conversion of the temperature gradient into electric current in the most effective possible fashion, the thermoelectric elements are assembled to constitute a module in such a way that the individual elements are connected electrically in series but thermally in parallel. Those modules can in turn be combined to form larger units (FIG. 7). Such an apparatus can be found for example in EP 339 715 A1.
The choice of the materials used is based on achieving the maximum possible efficiency in the desired temperature range. The efficiency is generally characterized by figure of merit Z=S2/ρκ (S is the Seebeck coefficient or absolute differential thermoelectric force, ρ denotes the specific resistance and κ denotes thermal conductivity). High efficiency is achieved in a material with a high Seebeck coefficient, a low specific resistance, and a low thermal conductivity.
The thermoelectric elements based on pairs of n- and p-type material blocks are those which in that respect are most developed, but have scarcely any advances to show even after more than 50 years of development time. Those thermoelectric elements can be obtained as a product for decades and are used in particular for cooling (thermoelectric cooler, Peltier module).
The essential advantage of that state of the art is that the production processes have been known for decades and are in a mature state. However, it also has the following disadvantages:
The properties of a material which are important for thermoelectricity (S . . . Seebeck coefficient, ρ . . . specific resistance and κ . . . thermal conductivity) can be influenced independently of each other only to a very slight degree. That relationship limits the efficiencies achievable at the present time to about 10-20% of the Carnot efficiency.
The configuration of the temperature gradient has no influence on efficiency as it is only the overall difference in temperatures between the hot side and the cold side that plays a part in conventional thermoelectric elements by virtue of the linear relationship between the thermoelectric force and the temperature difference.
Power density is too low to economically provide high power levels.
A further highly promising line of development in the boundary area of thermoelectricity and thermoionic effect has been followed by Eneco, Inc. of Salt Lake City, Utah, USA (Yan R. Kucherov and Peter L. Hagelstein), with the development of a thermoionic converter and a thermal diode.
A thermoionic converter (FIG. 8) comprises a heated metal plate and a cooled metal plate separated by a vacuum and an external circuit. Due to the higher temperature in the heated metal plate, more electrons have enough energy to overcome the potential barrier in the direction of the metal plate than in the reverse direction. In that way, current can be obtained from a temperature difference. It will be noted however that this process can only take place at very high temperatures because of the high potential barrier.
Thermal diodes have the same functional components, but the vacuum is replaced by a semiconductor. A diagrammatic structure of an n-type thermal diode from Eneco, Inc. is shown in FIG. 9. The semiconductor in place of the vacuum provides for a lower potential barrier and therefore the thermal diode functions even at lower temperatures.
With the correct arrangement of further potential barriers between the collector and the gap semiconductor, that arrangement prevents electrons from flowing back again. Accordingly, electrons are accumulated and a higher working voltage can be attained.
The advantages of the above-described configuration are as follows:
The efficiency of conversion of heat into current is higher.
In contrast to thermoelectric elements of the general kind set forth, thermal diodes can be connected in series without efficiency suffering therefrom. Accordingly it is easier to reach the maximum theoretical efficiency.
However, the disadvantages of this configuration are as follows:
This structure operates only with electrons, there is no thermal diode for holes, therefore the circuit has to be closed by way of an electrical conductor, by way of which heat also flows and therefore reduces efficiency.
The effect used only occurs if the thicknesses of the barriers are in the region of the leakage lengths and thus at some 100 nanometers (when InSb is used 1.5 micrometers). At higher temperatures the diffusion of materials is greater, therefore the potential barriers are rounded with time and the lengths necessary for maintaining the effect are no longer observed. The temperatures which can be used for current generation are therefore severely limited upwardly.
In order to utilize the generation of electron-hole pairs, a pn-junction with a temperature gradient can be utilized in known thermoelectric elements (AT 410 492 B).
In the structure shown in FIG. 10, electron-hole pairs are generated at the hot end as the thermal equilibrium between generation and recombination is shifted to the benefit of generation due to the charge carrier drift because of the temperature gradient. The pn-junction is a part of the overall structure which structurally cannot be separated from the location of the temperature gradient.
The advantages of the above-described configuration are as follows:
The operating temperatures can be extremely high depending on the respective material used.
Simple structure similar to a solar cell.
However, the disadvantages of this configuration are as follows:
The recombination of charge carriers cannot be entirely prevented so that efficiency is reduced.
Thick layers are necessary for the transport of charge carriers, and they make more demanding manufacturing methods necessary.
DE 101 36 667 A1 discloses the integration of a diode into an arm of a Peltier element.
GB 953 339 A1 discloses the structural combination of a Peltier element with a diode.